There are several reasons why a tennis player might desire to increase or decrease the gas pressure inside a tennis ball:                1. After the passage of time, tennis balls lose pressure, even though most of the felt covering is still on the ball. This causes the balls to have less bounce and velocity and for many players this makes the game of tennis less enjoyable. Many players simply discard these “lost pressure” balls and purchase new ones. A method to quickly and repeatedly re-pressurize the balls that have lost pressure will extend the playing life of balls, thus saving the cost of buying new balls, saving the resources used to make new balls and saving the pollution of disposing of the discarded used balls.        2. A newly manufactured tennis ball has a specified gas pressure, which causes the ball to have a specific bounce and velocity upon being struck by a racquet. A player may want to quickly increase the gas pressure inside the ball to increase the bounce of the ball and to increase the strike velocity. A player might do this in order to add more challenge, speed and excitement to the game, similar to some automobile owners modifying the “stock” engines of their vehicles to provide more power and speed. Other players may want to practice with “supercharged” balls to better prepare themselves for competition with other players who hit the balls with high velocity.        3. When playing tennis outdoors on cold days, balls have less bounce and velocity because the colder gas inside the ball exerts less pressure on the inside of the ball. A method to quickly increase gas pressure inside the ball will allow a player to have a ball with normal bounce and velocity on a cold day.        4. Some players may want to use a method that will quickly and repeatedly decrease the gas pressure inside a tennis ball. In examples 2 and 3 above, a player may want to do so after finishing the activity when the increased pressure was desired. Beginning tennis players may want to decrease pressure in new tennis balls so the ball is easier to control or have lower velocity when struck by the racquet, and then increase pressure in the same ball as the player becomes more skilled. In high altitude locations, the factory gas pressure of normal tennis balls makes the balls bounce higher and with more velocity, which some players do not like. Manufacturers do make high altitude balls for this situation, but such balls might not always be available for purchase and they cannot be re-used by a player who may travel to a high altitude area and then return to a low altitude area. Being able to quickly adjust the gas pressure will enable to player to use the same balls in high altitude, low altitude and everywhere between.        
Disadvantages of Current Methods.
There are existing methods that claim to increase the gas pressure inside tennis balls. A discussion of some of the methods is found in U. S. Patent Office document US20100307637. Some existing methods require tennis balls to be placed in a sealed container that has a higher gas pressure than the inside of the tennis ball. The marketers of these existing methods claim that, over time, the higher outside pressure gas will permeate the rubber of the ball and cause the inside pressure of the ball to increase. The disadvantages of this method include:                1. It does not work with some balls.        2. The sealed containers can be expensive, costing from $20 to $5000.        3. The process, even if it works, is time consuming. It can take several days or more for the gas pressure inside the ball to reach a satisfactory level.        4. Because some embodiments of this method involve a high pressure container, there is a danger of explosion and injury.        5. The apparatus for some of the existing methods cannot be operated by a lay person.        6. Some of the apparatus requires the use of a special gas instead of regular air.        7. These methods do not allow for decreasing the pressure inside a ball.        
There are some patents that have been issued for methods of injecting gas into tennis balls using a sharp syringe device that penetrates the rubber bladder of the tennis ball, similar to a hypodermic injection in a human. These methods can cause injury to the user, don't work well, and require new materials such as sealants for each injection. These methods do not allow for decreasing the pressure inside a tennis ball.